In the manufacture of hybrid integrated circuits the manner in which the crystals are connected to, i.e., mounted on, the substrate or base support is highly important. The most widely used method of connection of the crystal is by wire. This method is relatively simple and is suitable for all types of crystals. With this method, the semiconductor crystals are connected by means of a eutectic or adhesive or are securely connected in some other manner to the base support, and then the conductive wires are welded ultrasonically or are bonded by thermocompression. The method is obviously highly labor intensive and despite the labor required, the conductive connections are not dependable and have only a low mechanical capacity to carry charges. Also, the electric parameters of the connections are very high (so that the increase in switching frequency is limited) and these parameters can vary from part to part.
Mounting methods or processes are also known which provide for the use of crystals with spherical, columnar, bar-shaped or other specially shaped outputs or contacts. The manufacture of semiconductor crystals with such special contacts however is technologically complicated and costly Also, in the case of constructions of this type there are always problems with monitoring the quality of the electric connections, problems with heat exhaust or withdrawal (heat sinking) from the crystal and problems with the high ratio of low-quality goods produced by this method.
In order to avoid the aforementioned problems, a process is described in British Patent 21 30 794 wherein the crystals are first placed face down on an auxiliary substrate and are covered with epoxy resin by casting from the reverse side forward. Following hardening of the epoxy resin, the crystals in the resin are separated from the auxiliary substrate, and then multi-layer conductive strips are applied by means of photopolymer paste.
This method or process has the advantage that the contact surfaces are accessible on the front face of the crystal and production of the connections is relatively simple. The mechanical strength of the connections with the substrate or base support is good. The linearity of the HIC surface is also advantageous.
However, there are also drawbacks which stand in the way of wider use of the process. Some of these drawbacks include the poor heat exhaust or withdrawal provided and the ultrahigh frequency (UHF) characteristic which requires the use of undesirable, quite low frequencies. Further, the synthetic resins deteriorate rapidly, and the base support is of low mechanical strength. The polymer conductors also have remarkably poor mechanical and electrical characteristics or parameters. Finally, special photosensitive paste material must be used.
There are other known methods which work with base supports made of inorganic materials, for instance of silicon, anodized aluminum or ceramic (Japanese patent No. 56-48 23). This type of process generally involves the provision of through-holes or blind holes in the support, into which the crystals are subsequently introduced. From the reverse side of the crystal, glass, polysilicon or some inorganic dielectric substance is then coated on. The HIC formed in this manner is flat and can be provided by vacuum-vapor-metallizing with conductive strips directly to the contact surfaces of the semiconductor crystals.
One drawback of all of the methods is that it is very difficult to attain flat, horizontal or level borders of the material. As a result, these methods are not suitable for the production of multicrystal HIC. Also, the plurality of boreholes provided decreases the mechanical strength of the base support as well as the overall construction. Further, the production of such boreholes, which must be highly accurate in their location and dimensions, is very costly and cannot, as a practical matter, be automated. In addition, the HIC constructions are susceptible to cyclical temperature deviations.
Another known method is that disclosed in U.S. Pat. No. 3,903,509, entitled "Semiconductor-Thermoplastic-Dielectric Substance Process," wherein the semiconductor crystals are fastened to the base support, thereafter a plurality of columns, stanchions or electric piles are drawn galvanically upward to a desired height, a dielectric material is laid on as cover, this mounting is opened at certain points, and conductive strips are mounted on top.
The advantages of this method include the great mechanical strength and the high degree of reliability in the manufacture of the conductive connections. The drawbacks include a limited frequency range on account of the transfer columns from one layer to the other, the requirement of placing the semiconductor crystals with great precision on the base support and the need to maintain the planar geometry of the crystals and the columns, stanchions or electric piles, so that the use of crystals of various heights is also not possible. Also, very complicated galvanic procedures are required in order to guarantee the exact column height, and it is not possible to assemble a plurality of crystals closely adjacent to one another.
The prior art method which is perhaps closest in concept to that of the present invention is the method disclosed by Schmid and Melchior in "Coplanar Flip-Chip Mounting Technique for Picosecond Devices," published in the Journal Rev. Sci. Instrum. 55 (1984), No. 11, pages 1854-1858. This process involves the steps set forth below which are described in relation to FIGS. 1 to 4 of the drawings.
First, as shown in FIG. 1, ultrahigh frequency (UHF) conductive strips CS1 and CS2 are provided, respectively, on a substrate or foundation (base) support S and a semiconductor crystal C. Then, as shown in FIG. 2, solder S0 and fluxing medium FM are applied to the substrate or base support. Subsequently, the single crystal C is mounted, as illustrated in FIG. 3, on the base support S with the contacts CS2 located on its front face. Finally, as shown in FIG. 4, the soldering takes place in a reducing atmosphere to produce the article illustrated.
Considering this process in more detail, an optical electronic structural element (C) of semiconductor crystal GaAs with coplanar (located in the same plane), tapering conductive strips (CSR) thereon is laid, with the side on which these strips are located, on a sapphire substrate or base support (S). The base support (s) is likewise provided with tapering, coplanar conductive strips (CS1). A 4 micrometer (mcm) thick indium layer has been applied on all of the contact surfaces of the base support beforehand by electrochemical means, and then a thin colophonium (resin) layer has been applied. The crystal is brought into the correct position under a microscope. The colophonium series as adhesive. Then in a H.sub.2 atmosphere the soldering occurs at 250.degree. for 30 seconds. In this process the colophonium serves as the fluxing medium. Then the colophonium residue is removed with acetone, and the crystal is surrounded with silicon rubber for protection.
Some important advantages of this known process are the relatively great mechanical strength and the suitability of the conductive connection between the crystal and substrate for the UHF-range. Thus it is possible to connect semiconductor crystals in the order of magnitude of a few microns to other semiconductor crystals in the order of magnitude of a few 100s or 1000s of microns. It is also advantageous that no holes need to be drilled in the substrate. On the other hand, the same drawbacks are present as in the prior art "flip-chip" method, which drawbacks include: the heat exhaust or drawing off occurs only through the soldered points or seams, the checking of the solder connections is made more difficult, the soldering requirements and conditions are complicated, special solder-activating components are needed, and the metal coating on both parts are not easily solderable. Also, complete removal of the colophonium between foundation support and crystal is not really possible, and the mounting surface of the crystal must be considerably larger than the surface of the active element, so that, in other words, only a slight benefit arises from the use of costly semiconductor materials. In the cited example above having, for instance, a surface of the active element smaller than 50 mcm.sup.2, a crystal is used with a mounting surface of at least 4 mm.sup.2 with the result that only one eighty-thousandth of the surface of the costly GaAs crystal is used. Finally, the problem exists that moisture from the air collects in the narrow gap between the substrate and the crystal, thereby negatively influencing the electrical properties of the connection and thus accelerating metal corrosion, which then can lead to breakdown of the structural element.